When an electrical storm suddenly hits, Will and Dr. Smith take refuge in a nearby cave. The storm causes the entrance to collapse, trapping them inside. The Robot rushes back to the Jupiter 2 for help, and the Robinsons try to dig Smith and Will out. Meanwhile, inside the cave, Will decides to look for another exit. He comes across a mysterious portal and passes through.
Will arrives a large, gloomy castle where he encounters a shrieking monster that chases him into the castle. There Will takes refuge in a dungeon. A number of frightening occurrences follow, culminating in the appearance of the ghost haunting the castle, Hamish, the late laird.
When the Robinsons finally open the entrance to the cave, they are shocked to find only Smith inside. Dr. Smith himself appears confused and maintains that he last saw Will walking down a passage—but there are no passages to be found. Don suggests that the electrical storm must have caused a portal to open between worlds and Will might have passed through. With the proper instruments, he thinks they could reproduce the storm and cause the portal to reopen.
Back in the castle, Hamish tells Will that he was executed in that very dungeon on a false charge of treason. Because of his shameful death, he was condemned to walk the earth as a spirit. He explains that the monster Will encountered is his uncle Angus, and that only Hamish’s bagpipes can soothe him when he gets upset. Hamish finally sends Will away, and Will finds his way back through the portal into the cave.
The Robinsons are relieved to have Will back, but they doubt his story about a haunted Scottish castle. Smith, though, is intrigued at the prospect of going back to Earth. Determined to prove his claims, Will slips out of his bed that night and heads to the cave. There he finds Hamish, restored to flesh and blood having passed through the portal.
Now that they know the portal leads back to Earth, the Robinsons believe they can go home at last. The portal is limited, though, so only one person, Dr. Smith, is to return with Hamish. Dr. Smith is to contact Alpha Control in order to effect the Robinsons’ rescue. Once he reaches Earth, though, he abandons the plan and Hamish disgusted. When he finds out that Smith is descended from the family that engineered his execution, Hamish turns him over to a spectral headsman for punishment.
At that point Will arrives and begs Hamish to spare Smith. When the ghost refuses to listen, Will runs to Angus for help. Angus nearly attacks Will, but Hamish intervenes and chases the monster away. Hamish then allows Smith and Will to pass through the portal back to the planet where the Robinsons wait. As reward for showing mercy even to a blood enemy, Hamish and Angus are given their eternal rest.
- Some source give this episode the title "The Astral Traveller", but the actual title is "The Astral Traveler", with one "L".
- The "dimensional gateway" looks suspiciously like a standard revolving door.
- This is an episode that strays very, very far away from the original series format but nevertheless remains a charming and entertaining fantasy if taken on its own merits. A "dimensional gateway" that directly connects to Earth appearing in a cave in which Dr. Smith and Will are trapped begs enough credulity without adding the elements of its terminus being a medieval Scottish castle where the only inhabitants are a swamp monster and a ghost (in some respects this plot was copied in the Space:1999 episode "Journey To Where.") However Sean McClory is a delightfully hammy Hamish and for once the monster is a sympathetic character
- How is Don able to immediately deduce that Will slipped through a portal?
- Why would the Robinsons trust someone as unreliable as Smith with their recue?