While searching for a sunken wreck, Mike's hard hat diving partner disappears, leaving nothing behind but his torn air hose attached to their boat on the surface. ...

An inventor dies while testing a new underwater detection device. ...

Mike's assignment: Retrieve a deadly radioactive capsule and restore it to a lead container without exposing himself to gamma radiation that could take his life. ...

The Enterprise responds to a distress call from the planet Scalos, but when Kirk and a landing party beam down to the planet they find no living beings. It turns out that ...