Following the defeat of the Rock Trolls, Thor is commended for his contribution to the battle. After the celebration ceremonies, Odin retires to the Pool of Peace for relaxation. There he is visited by a troubled Thor, asking permission to return to Earth as he fears that something may be happening in his absence.

This proves to be a prophetic concern as back on Earth, a scientist has discovered a strange doll size creature that has been buried for millions of years and yet appears to be made from materials from the future. Stranger still, this doll-sized humanoid begins to grow at an alarming rate. When The scientist shows his discovery to his superiors, they attempt to test its composition causing the Growing Man to revive and begin going on a rampage throughout the city, increasing in size as he goes about his masters' bidding.

While back on Asgard, Odin grants Thor's request to return to Earth, and when Sif asks if she may join the Thunder God, Thor declines to advise her that he is doing "man's work" before Odin transports him back to Earth. Appearing in New York City, Thor returns to his office and reverts to his mortal guise of Donald Blake. There Blake reflects on his dual role in life when he is interrupted by the office janitor, who asks Blake if he knows when Jane Foster will return. Unfortunately, Blake has no answer.

Elsewhere in a suburban park, the Growing Man has led the police on a merry chase of wanton destruction and continues to overpower them. Suddenly, the Growing Man's master reveals himself as the future tyrant Kang the Conquerer, who uses a special device to shrink the Growing Man back down to a miniature size.

In desperation, the police pick up Donald Blake, who they know has ties to Thor, and bring the doctor to the scene in hopes that he can call Thor to stop Kang. When the police leave Blake unattended, the doctor taps his cane and transforms into Thor.

Kang has once more let loose the Growing Man, and Thor manages to hold his own against the foe despite its ever-increasing size. This is until Kang gets the drop on Thor from behind and manages to overpower the hero with his advanced future technology.

Kang explains to Thor that he had unleashed the Growing Man on 20th Century Earth as a test of the creature's abilities as he plans on using the synthetic creation to take over worlds in the future. Satisfied with the results, he reduces the Growing Man back down to doll size and tries to escape a time machine hidden in a rock formation.

Not wishing Kang to escape with such a weapon, Thor uses his hammer's mystical energies to trap the time machine in a time vortex from which it can never escape. In a flash of light, Kang's time machine vanishes, and Thor believes that his enemy is vanquished for good.

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