NOTE: This spoiler was submitted by Jeremy

The film starts on the planet Krypton just before its destruction. Jor-El (Alfred Molina) and Lara (Lena Headey) send baby Kal-El away in a pod, tearing him away from his best friend, Krypto, a Labrador puppy. Before the pod closes, Krypto hops inside to keep Kal-El safe. Although Jor-El tries to stop them, Lara tells him that it will be better for their son to have a friend. The pod flies away as Krypton blows up. Krypto comforts a crying Kal-El.

In the present day, everyone knows Kal-El as Clark Kent/Superman (John Krasinski). Krypto (Dwayne Johnson) is still his best friend, and they do many activities like flying around Metropolis and beating up bad guys. However, Krypto is not good with other animals, as shown when he interacts poorly with other dogs at the park. He also gets jealous at Superman’s relationship with Lois Lane (Olivia Wilde). The other dogs tell Krypto that he might be left out now that Superman has a girlfriend.

Clark takes Krypto to a pet shelter to try and find him a new friend. In the shelter are animals that think they will never be adopted – a Boxer dog named Ace (Kevin Hart), a pig named PB (Vanessa Bayer), a turtle with poor eyesight named Merton (Natasha Lyonne), a squirrel named Chip (Diego Luna), and a bald guinea pig named Lulu (Kate McKinnon), who was experimented on by Lex Luthor (Marc Maron) and now has plans for world domination. Ace makes an attempt to escape the shelter and climbs out the window. He meets Krypto outside, where they do not get along as Krypto freezes Ace’s feet on the pavement, leading to him getting taken in again.

Later that day, Krypto and Superman use their x-ray vision as they find Lex and his assistant Mercy Graves (Maya Erskine) trying to pull in a meteor of orange kryptonite using a tractor beam, as Lex thinks it will grant him superpowers. Krypto and Superman head off to stop him and are joined by the rest of the Justice League – Batman (Keanu Reeves), Wonder Woman (Jameela Jamil), Aquaman (Jemaine Clement), Jessica Cruz/Green Lantern (Dascha Polanco), and Cyborg (Daveed Diggs). Together, they defeat Lex and have him and Mercy arrested. However, a small shard of the orange kryptonite gets pulled toward the shelter by Lulu, as she tells the others that the kryptonite works on animals. She is given powers and breaks free from her cage as the shelter starts to catch fire. The other animals get powers too, with Ace becoming super strong and invincible, PB being able to change her size, Merton becoming super fast, and Chip getting lightning powers. They run from the shelter in time.

Clark later gets ready for a date with Lois, while Krypto wants to watch “Great British Bake-Off” with him. Krypto fantasizes about throwing Lois far away but instead protests (ie, barks a lot) to Clark, who is fed up with Krypto’s complaints and calls him a bad dog. Hurt, Krypto retreats to his room and drowns out the noise with music. As Clark steps outside, he is greeted by Lulu, who holds up green kryptonite to him and weakens him. He gets thrown out into the air, which Lois notices, so she calls the rest of the League. Krypto finds out what is wrong and tries to spring into action, but after he eats a piece of cheese hidden in a Batman dog toy, he becomes depowered because Lulu put a piece of kryptonite in the cheese.

The rest of the Justice League arrives to fight, but Lulu has already amassed an army of guinea pigs and given them their own powers. The guinea pigs overpower the League and capture them as well. Krypto runs into the other shelter animals and asks for their help in getting Superman back. They say no at first, because they want to go to a farm that Ace keeps mentioning, but when Krypto realizes he is lying to them, he gets them to help by blackmailing Ace so he won’t tell them the truth.

The pets catch up to Lulu as she heads to Stryker’s Island to free Lex. They are held off by Lulu’s army, as well as a kitty named Whiskers (Winona Bradshaw) that Lulu saved from the shelter. Whiskers coughs up hairball grenades and fires rockets from her tail. Lulu defeats them because they can’t keep their powers under control, and Krypto gets thrown toward the Hall of Justice. He feels defeated until he is joined by Ace. He tells Krypto about his backstory, where he used to be owned by a loving family until he bit the baby’s arm when trying to save her from falling down the stairs, leading to him being sent to the shelter. Ace tells Krypto this is just how dogs love unconditionally. They rejoin the other pets to continue their mission.

Krypto helps the pets work together to hone in their powers to fight Whiskers. PB learns to be more self-confident, Chip learns to not be so stressed out, and Merton gets glasses. Together, they trap Whiskers under a dumpster where she gets blown up by one of her grenades (but it’s okay because she still has eight lives).

Lulu makes it to Stryker’s Island and takes out all the guards before making her way to Lex’s cell. Although Lex appears happy to see Lulu, he betrays her and throws her into his cell after he is freed. He takes Mercy with him and they trap the other pets in the cells once they show up. Krypto tries to get through to Lulu and offers to let her join their team so they can save the Justice League, but she instead grows angrier and decides to take out Lex and the League on her own. She breaks out of her cell and heads off. Krypto suddenly regains his powers after he, erm, passes the kryptonite, and he helps his friends escape, but he tells them to stay behind, so they don’t get hurt. Of course, they don’t listen to him.

Lulu catches up to Lex and traps him along with the League, and she places them all in a rocket that will explode once it breaks past Earth’s atmosphere. Krypto finds them and fights Lulu and the other guinea pigs. Lois also gets caught in the chaos when her chopper is hit by Lulu. Krypto goes to save her while the other pets rush to save the League. PB grows herself big enough to have Merton run up with Chip and Ace to the top of the rocket. Chip fries a hole so Ace can use his strength to free the League (and Lex). Krypto fights Lulu and breaks her kryptonite rock, but instead of depowering her, she absorbs the kryptonite into her brain, transforming her into a gigantic kaiju. The newly freed League fights Lulu along with the pets, but she is way too powerful. Krypto then decides he has to do the Solar Paw Punch, a move involving him flying all the way to the sun even though it might kill him. He does so and punches Lulu hard enough to remove the kryptonite from her body. Once she returns to normal size, two of her guinea pigs, Mark and Keith (Thomas Middleditch and Ben Schwartz), trap her in a hog dog stand. Ace then saves Krypto from falling to his death.

Krypto reunites with Superman, and he gives him the ring he was going to use to propose to Lois, and she accepts. The other animals are taken in by the League – Wonder Woman adopts PB after considering her a mighty warrior, Jessica takes in Chip as part of the Green Lanterns, Flash adopts Merton, and Mark and Keith, who respectively have fire and water powers, go with Cyborg and Aquaman. Batman and Ace pretend to be brooding loners, but they stick together.

The Super Pets become an official team on their own. Their first enemy is a corgi that Krypto met earlier which has now grown to colossal size. The pets head off to save the city once again.

Mid-Credits Scene: Mercy has gotten Lex’s suit of armor and finds Lulu. She takes her in as she knows they are both the brains behind Lex’s success, and they head off to make their own evil plans together. Meanwhile, Lex is still stuck in his bubble.

Post-Credits Scene: Krypto and Superman are playing in the park when they are found by Black Adam and his dog (both played by Dwayne Johnson). The other dog tries to convince Krypto that Adam is an antihero despite describing him as a villain. Krypto then makes both of them go away by throwing a dog toy.

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Krypto the super dog is Superman's best friend, but he has trouble getting along with animals and letting go of Superman when he is dating Lois Lane. Meanwhile, a guinea pig named Lulu that was experimented on by Lex Luthor obtains orange kryptonite that gives her superpowers, along with other shelter animals Ace the dog, PB the pig, Merton the turtle, and Chip the squirrel. Lulu imprisons the Justice League with the help of her own guinea pig army that she grants superpowers, and she tries to free Lex from prison.

Krypto teams up with the other pets to save the League, and he helps them work together to keep their new powers under control. After Lulu frees Lex, he betrays her and goes off with his assistant Mercy Graves. Lulu becomes enraged and decides to kill Lex and the League by launching them in a rocket that will explode upon leaving Earth's atmosphere. The Super Pets work together to free the League and fight Lulu, but she absorbs the kryptonite to turn herself into a gigantic monster. Krypto uses a special move to defeat and depower her and Ace saves him.

Krypto accepts Superman and Lois getting engaged, and the League adopts the rest of the pets, plus two of Lulu's reformed guinea pigs. The Super Pets become their own heroic team, but Mercy later finds Lulu so they can work together on their own evil schemes.