The paper presents the implementation of a tool for plagiarism detection developed within the AXMEDIS project. The algorithm leverages the plagiarist behaviour, which is modeled as a combination of 3 basical actions: insertion, deletion, substitution. We recognize that this behaviour may occur at various level of the document structure: the plagiarist may insert, delete or substitute a word, period or a paragraph. The procedure consists in two main steps: document structure extraction and plagiarism function calculation. We propose a recursive plagiarism evaluation function to be evaluated at each level of the document structure which is based on the Levenshtein edit distance. We also propose a method that will eliminate unnecessary chunks comparison, avoiding similarity calculation of chunks which do not share enough 4-grams. We describe the similarity algorithm and discuss some implementation issues and future work.