The Fifth Doctor (Peter Davison) discovers that his attempt to return Tegan (Janet Fielding) to Heathrow Airport has brought the Tardis to a spacecraft that appears abandoned at first, but actually carries a force of frog-like aliens that may challenge the future of humankind on Earth. Also featuring Nyssa (Sarah Sutton) and Adric (Matthew Waterhouse)

The Fifth Doctor discovers that his attempt to return Tegan to Heathrow Airport has brought the Tardis to a spacecraft