The completed grid can be found here.
To solve this puzzle, one first fills in the crossword; doing so involves several squares whose contents are, rather than the usual one letter, a number between 1 and 9, which can be read either as the number or the letters that make it up. After finding all of these, one also has to solve the embedded Sudoku puzzle. Finally, one reads 9 down, which suggests how to extract answers: first, one reads the letters placed where 6 is placed in the solved Sudoku, which spells out SURFER. Next, one looks at all instances of the letter S in the grid where a clue entry number is written. Reading these numbers as letters (1–26) spells out the other answer, ARSENIC.