I kept mixing this up with the K18 leave-in sitting next to it. Olaplex N°.3 PLUS goes on before you shampoo, and a month of weekly use in the right slot took the frizz out of hair that had gone brittle on me.
Worth the money if your hair has gone brittle and frizzy with age and you will actually keep to a weekly step. It did what two rounds of heavy oils could not, and it kept doing it week after week for over a month. Buy it with the order straight in your head, before the shampoo, expecting weeks rather than days, and counting on rebuying it.