Britain's Samantha Murray set a 200m freestyle record on her way to World Championship gold in Poland.
Murray, 24, was back in 14th after the fencing, but produced an incredible swim of two minutes 2.84 seconds - the fastest ever by a modern pentathlete.
After a solid showjumping round she then hauled herself to victory in the final run-shoot combined event.
The result, combined with that of team-mates Freyja Prentice (19th) and Kate French (26th) saw GB claim team silver.
Murray, silver medallist at London 2012, is the fourth British woman to win the title, following Wendy Norman (1982), Steph Cook (2001) and Mhairi Spence (2012).
World victory, with 1,411 points - ahead of Chinese duo Chen Qian (1,403) and Liang Wanxia (1,384) - marked a dramatic turnaround in form for the Bath-based athlete after seventh- and eighth-place finishes at the last two European Championships.
"It means so much to me," Murray told BBC Sport.
"This morning I literally had a nervous breakdown and my fencing was all over the place but I came through that, I started smiling and relaxed, then everything came together.
"This is a result of all the work I've put into my sport and I deserve this title today."
The team silver continued an impressive run for the British women, who have won a medal in the event at each of the last three World Championships.
"Pentathlon has a good history in Britain and we're trying to continue that," Prentice told BBC Sport.
French added: "When I knew I was out of the running for an individual medal it was all about getting my head down and doing the best I could for the team."
Saturday will see Britain's Jamie Cooke and Joe Choong compete in the men's final.