Mercurial and kaleidoscopic in its presentation, defying categorisation, postpartum psychosis is the will o’ the wisp of mental illnesses. Affecting one in 500 women, its onset is early, abrupt and dramatic. Confusion, perplexity and mood lability are typical but little is certain in this most elusive condition. What is predictable is the risk of recurrence, as high as one in two following subsequent pregnancies. No other psychiatric malady can be predicted with such certainty, offering unique opportunities for prevention. We may even be close to the essence of this chimera through increasing evidence of its close link with bipolar disorder.