In Portal, our heroes are stranded on Europa after the events of Threshold. While they and the survivors from the Odin attempt to jury-rig a way to get home, they discover that General Hohenheim is still alive and work out a way to rescue him. (They are also making an effort to keep what happened more or less secret. There is a very real concern about further attempts at sabotage or murder from the enemies who instigated the events of the previous book.) Meanwhile, back on Earth, there is a growing suspicion that the accident wasn’t one.
Despite the very desperate situation Our Heroes find themselves in, they are not the types to avoid doing Science when there is Science to do. In between the ongoing “find a way to get home” project they do some exploring that leads to a number of shocking discoveries and also some terrifying accidents that lead to still more discoveries. (It is of course no surprise that Joe Buckley is involved in the terrifying accidents. The universe still seems determined to either scare or beat the crap out of him. Of course, this might be an improvement over universes where the universe is actively attempting to kill him.)
This is an extremely fast-paced, engaging novel with some fun character interactions and a strong sense of adventure. The only thing I didn’t like about the book is that it felt much too short. I think I would have liked more details about some of the discoveries, or at least some more development of the various character arcs. This is another transitional book in the series that leaves you waiting for more.