Clara Morgan sends her children, Tommy and Mary, off to school. They meet up with their cousin, Ralph, and the other children for the long walk to school. On the way, they are surprised to see a man jump over a hedge into the lane. He is followed by Mr Wesley, the mill owner, who chases the stranger but cannot keep up. He comes back and yells at the children before returning to the mill.
Further down the lane, the children see the stranger, who introduces himself as the Doctor. The Doctor heads for the bog, about which the children have been warned. Tommy and Ralph decide to follow the Doctor and send the other children along to school.
The Doctor is using a tracking device, and he explains to the boys that he is looking for an alien, and that he himself is an alien. Tommy and Ralph want to help. The Doctor keeps to himself his worries that aliens might attack Earth if their missing people can't be found. The Doctor's device leads them to a crashed Velderon spaceship. The Doctor rescues one Velderon, wrapping it in his jacket. He explains to the boys that there should be another one, that Velderons don't react well to water, and that the ship will sink into the bog.
As they head towards the mill, where the Doctor has stashed his TARDIS, they hear a scream. The Doctor gives the Velderon to Ralph, and Tommy and the Doctor head for the sound. It is the minister's wife, who is speechless in terror but manages to direct them to the source of her terror.
Meanwhile, Ralph notices that it is about to rain and that the Velderon's feet are exposed. He wraps its feet with his scarf, and heads for the mill.
The Doctor and Tommy find the other Velderon, who is terrified of them and the onset of rain. It backs away from them, despite the Doctor's calming voice. As it backs away, it falls into a well. When its body hits the water, there is an explosion of acidic gas.
Ralph is discovered by George Evans-the-Cwm, but he manages to send the man away and hides in his and Tommy's secret den. The Doctor, now worried more than ever about a possible Velderon invasion, takes Tommy to find Ralph. When they find Ralph, he has been feeding the alien.
The Doctor has a plan: he will distract Mr Wesley, and the boys will take the Velderon to the TARDIS in the mill. Mr Wesley has an angry conversation with the Doctor until he spies the boys heading for his mill. He heads for them, but the Doctor tackles him and the four arrive at the TARDIS. As the Doctor and the Velderon prepare to leave, the Velderon gives Ralph his scarf.
Tommy and Ralph head home, where Tommy's mother can't find it in her to scold them for skipping school.
- Tommy has read The First Men in the Moon by H. G. Wells.
- Ralph feeds the Velderon part of his cheese sandwich.
- All the stories in Short Trips: Past Tense take place some time in Earth's past. This story takes place in 20th century Wales.
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