A Papua New Guinea victory could reshape the Rugby League World Cup knockout race and make next weekend's match against England in Wollongong a semi-final qualifier.
The Kumuls, however, trailed Samoa 14-6 at halftime in their Week 2 clash at Santos National Football Stadium in Port Moresby.
Samoa took an early 4-0 lead after a wayward short kickoff gave Knights forward Franicis Manuleleua a chance to charge downfield. The Kumuls stopped him, but Storm winger Sua Fa'alogo scored at the end of the set.
PNG responded with strong defence and repeated pressure before veteran Rhyse Martin scored in the 18th minute, putting the hosts ahead 6-4.
Robert Derby appeared to score three minutes later, but the bunker ruled he had knocked on.
Samoa regained the lead through Spencer Leniu in the 30th minute. A second apparent Leniu try was overturned because he had grounded the ball just short of the line.
Stephen Crichton caught a loose kick and scored in the corner four minutes before the break, extending Samoa's advantage.
The physical first half ended in a brawl after Leniu tackled Kumuls five-eighth Gairo Voro heavily.
PNG needed to lift again after halftime to complete the upset.