Big Fish is the 10th broadcast episode and 7th produced episode of British television series Joe 90.
An atomic submarine malfunctions in enemy waters, and only Joe 90 can save it before a political frenzy breaks out.
An advanced two-man submarine is damaged through a missile outlet fault which allows an inrush of water. The crew are rescued by helijet, but unfortunately the sub has drifted into forbidden waters and now lies in Porto Guavan territory. This threatens a tricky international situation unless the sub can be moved without discovery. Joe 90 is therefore given the brain patterns of the world's leading aquanaut, and he and Professor McClaine visit Porto Guava, ostensibly on a fishing trip. But they attract the suspicions of a boatman, from whom they hire his craft. The Porto Guavan Coastal Control is also suspicious, and sail out to make inquiries just as Joe is under-water, trying to get into the stricken sub. To make matters worse, he is trapped. The authorities, believing that he must have been deliberately drowned, arrest the Professor and the boatman on suspicion of murder. Joe, however, succeeds in freeing himself and also getting the sub safely away from the forbidden area before arriving at the jail to prove that he certainly hasn't been murdered and that there is no case against his father and the boatman!
- The hotel where the McClaines spend their time in Porto Guava is reused from the Thunderbirds episode Lord Parker's 'Oliday.
- The Lieutenant was previously Doctor Fawn in Captain Scarlet and the Mysterons, while one of the prison guards is portrayed by Fawn’s Spectrum colleague Captain Magenta.
- When the U85 sinks, the jammed torpedo tube is on the starboard side, but when Joe dives to the sub, the jammed tube is on the port side. As Joe approaches the U85, the markings are in mirror image in long-shot.