Gary comes across a magazine quiz on how to tell if your partner is cheating. All seems strangely accurate when Dorothy comes round with flowers and gifts and is evasive about holiday plans. Meanwhile, Debs is finding Tony increasingly tiresome and tells him that, although he is good looking, he is far too selfish for her to be interested in him.

Jealous, Gary decides to follow Dorothy and takes pictures of her as she does her chores and works as a hospital nurse. Tony volunteers as a helper in an old folks' home but bores them all by talking about Debs all the time and performing a very unfunny cabaret routine. Debs remains unimpressed.

Dorothy comes to Gary's flat only to be confronted with a slide show where Gary, armed with a pointing stick to illustrate clues, announces his suspicions. Dorothy confirms that she is seeing someone else and dumps Gary, who is aghast that his worries have turned out to be true. Tony is also downcast at finding that Debs now has a boyfriend.

Gary and Tony bitterly drown their sorrows as they watch slides of Gary and Dorothy's five years together. Meanwhile Debs and Dorothy toast the prospect of their new relationships.