You gave too much rein to your imagination. Imagination is a good servant, and a bad master. The simplest explanation is always the most likely.

The remark cautions against letting speculative fancy outrun evidence. By calling imagination a “good servant” but a “bad master,” the speaker distinguishes creative hypothesis-making (useful for generating possibilities) from ungoverned conjecture that dictates conclusions. The final sentence aligns with a common detective-story ethic: prefer the least complicated account that fits the facts, rather than baroque theories driven by drama or wishful thinking. In Christie’s world, this stance often functions as a rebuke to amateurs who romanticize crime, and as a reminder that clear observation and disciplined reasoning—not sensational narratives—most reliably lead to truth.